The invention relates to a device for balancing an energy accumulator (store), in particular an energy accumulator in a motor vehicle's onboard power supply system, which consists of a cell stack composed of the series connection of a plurality of groups of cells, wherein a respective balancing circuit is connected to voltage terminals of a respective group of cells and comprises a discharge resistor.
A growing trend in the automotive field is to use energy accumulators constructed of individual cells, preferably energy accumulators constructed of double layer capacitors. For providing and storing energy, these double layer capacitors offer the advantage of being able to provide a lot of power in a short period of time. In order to attain the amount of power supply voltage that is required in a vehicle, the individual double layer capacitors have to be connected in series. In this case the series connected cells form a cell stack. For charging, for balancing the cell voltage, for detecting the undervoltage and overvoltage of individual cells, for maintaining the charge of the cells in the stationary operation, for the charging and discharging or the charge reversal between the individual cells, as well as for the diagnosis/monitoring of the individual cells, it is necessary to monitor individual cells or groups of cells. It is known that such monitoring must be provided with cell group logic that is connected to a group of cells in order to monitor and/or control or regulate its voltage. In the context of the invention, a group of cells can also comprise just a single cell. So that the cell voltages of the individual cells or groups of cells do not diverge during the cyclical charging or discharging, it is known to monitor the cell group logics by way of a higher ranking automatic control device, for example a central logic, and to drive it in such a way that a uniform charging and/or discharging is achieved. This approach makes it possible to attain a long service life of the individual cells and, thus, of the entire cell stack.
WO 2007/104325 A1 discloses a diagnostic circuit for an energy accumulator. Upon overshooting and undershooting the voltage of a cell group, the diagnostic circuit impresses the same levels on a diagnostic rail, which can then be used to interrogate a monitored operating parameter of the cell group and/or the cell stack. The described diagnostic circuit is also used for active cell balancing—also equilibration—of the accumulator cells. The balancing is based on the principle of executing a voltage matching among the individual cells or groups of cells of the cell stack. For this purpose, a balancing circuit is connected to voltage terminals of a respective group of cells. The voltage is monitored by a comparator of the balancing circuit that is coupled to the voltage terminals. If a specified voltage value is exceeded in a group of cells, then the voltage terminals of the respective group of cells are short circuited by means of a discharge resistor. This discharge resistor can be used to discharge the group of cells.
The rate of balancing depends on the discharge capacity of the discharge resistors that are used in the balancing circuits. In order to attain the shortest possible discharge time, the resistors have to be dimensioned as small as possible in size. Then, however, the result is a very high heat loss that has to be dissipated in a suitable way. In an energy accumulator with a plurality of series connected cell groups, the balancing circuits, which are assigned to the respective cell groups, including the respective discharge resistor, are mounted on a printed circuit board (PCB). Since a printed circuit board cannot dissipate the heat loss in infinitely large quantities, the resistors are designed as a high impedance type, usually between 10 and 100 ohms. As a result, however, the balancing rate is restricted.
Therefore, the object of the present invention is to provide a device that is intended for balancing an energy accumulator and that makes it possible to balance the groups of cells of the energy accumulator in a simple and fast way.
This and other objects are achieved with a device according to the invention for balancing an energy accumulator, in particular an energy accumulator in a motor vehicle's onboard power supply system, which consists of a cell stack composed of the series connection of a plurality of cell groups. A respective balancing circuit is connected to voltage terminals of a respective group of cells and comprises a discharge resistor. The discharge resistor of a respective balancing circuit has a temperature dependent resistance characteristic. Furthermore, the discharge resistors of at least some of the balancing circuits are thermally intercoupled.
In particular, it is provided that the resistance value of a respective discharge resistor increases with a rise in temperature. Therefore, a respective discharge resistor exhibits advantageously a PTC characteristic. PTC stands for positive temperature coefficient. PTC resistors or, more specifically, PTC thermistors can conduct a current better at lower temperatures than at high temperatures. For this reason their electric resistance increases as the temperature rises. Thus, PTC resistors exhibit a positive temperature coefficient.
According to an additional embodiment, the discharge resistors of a respective balancing circuit are thermally intercoupled. In particular, it is provided that for the thermal coupling of the discharge resistors, they are mounted on a common heat sink. The heat sink is made preferably of a material that conducts heat well, in particular a metal. For practical purposes all of the discharge resistors of all of the balancing circuits of the devices are mounted on a common heat sink. The result is a thermal coupling of the discharge resistors.
In the event that the discharge resistors of the individual cell groups do not operate simultaneously, the amount of thermal energy that can be dissipated relative to an individual group of cells is larger, so that it is possible to achieve a fast discharge of the cell group to be discharged. If a plurality of cell groups of the energy accumulators is discharged simultaneously by way of the respective discharge resistors for the purpose of balancing, then after a period of time the thermal absorption capacity of the heat sink is reached, so that its temperature rises. Then, the temperature of the respective discharge resistors also rises, as a result of which the discharge time may increase, but at the same time the heat loss is reduced.
Consequently, the temperature dependent resistance characteristic of the discharge resistors provides a component protection function. If only one discharge resistor of a balancing circuit is active, then this balancing circuit can use the maximum cooling capacity for the cell group to be discharged. This feature provides a high discharge capacity. Then this higher discharge capacity results in an accelerated balancing—that is, a balancing that is shorter in terms of time. In addition, the procedure according to the invention makes it possible to minimize the dimensions of the heat sink.
In another practical embodiment, a respective balancing circuit is designed as an active bypass balancing circuit. Such active bypass balancing circuits are known, in principle, from the prior art and are disclosed, for example, in the introductory part of the aforementioned WO 2007/104325 A1, the entire disclosure of which is expressly incorporated by reference in the present patent application.
Furthermore, it is provided that a group of cells includes at least one accumulator cell, in particular a double layer capacitor.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.